1. Field of the Invention
The present invention relates to an electric circuit using a surface acoustic wave device. More specifically, the present invention relates to an electric circuit such as a video intermediate frequency signal amplifying circuit of a television receiver using a surface acoustic wave filter.
2. Description of the Prior Art
FIG. 1 is a view showing one example of a conventional surface acoustic wave device. As well known, the surface acoustic wave device SAW comprises a substrate 1 made of a piezoelectric material such as piezeoelectric ceramic, and an input transducer 2 and an output transducer 3 formed thereon each including an electrode arrangement of such as an interdigital pattern. The input transducer 2 comprises paired comb-shaped electrodes, so that the electrode fingers 2a and 2b of the respective comb-shaped electrodes are adjacent to each other. Similarly, the output transducer 3 also comprises paired comb-shaped electrodes, so that the electrode fingers 3a and 3b are adjacent to each other. One comb-shaped electrode of the input transducer 2 is connected to the input terminal 4 and the other comb-shaped electrode is connected to the input terminal 4'. Similarly, one comb-shaped electrode of the output transducer 3 is connected to the output terminal 5 and the other comb-shaped electrode is connected to the output terminal 5'.
Such a surface acoustic wave device as a surface wave filter is employed in a circuit configuration as shown in FIG. 2. More specifically, a signal is supplied from a signal input circuit 10 including a signal source 11 and having an output impedance whose resistive component is denoted by R1 to an input transducer of a surface acoustic wave device SAW. An output transducer of the surface acoustic wave device SAW is connected to a load 21 of a signal output circuit 20 having an input impedance whose resistive component is denoted by R2. Referring to FIG. 2, inductances L1 and L2 are well known tuning coils.
Component characteristics in such a circuit configuration as shown in FIG. 2 must be such that the amplitude frequency characteristic of the circuit is good, the group delay time characteristic is good, the insertion loss is small and so on. A ripple in the pass band in the above described amplitude frequency characteristic and group delay time characteristic is susceptible to an electric triple transit echo (which may be referred to for brevity as TTE) of the surface acoustic wave device SAW. The electric triple transit echo and insertion loss are determined by the relation between the equivalent resistance component of the input transducer or output transducer of the surface wave filter SAW and the resistance component R1 or R2 of the signal input circuit 10 or the signal output circuit 20. Thus, in order to decrease the electric triple transit echo and to improve a ripple in the pass band, it is important that the resistance component proper of a transducer be made large as compared with the output impedance (resistance) or the input impedance (resistance) of an external circuit. To that end, one might think of relatively increasing the resistance component proper of a transducer or of relatively decreasing the resistance component of the external circuit. Employment of a transformer is known as the approach to decrease an electric triple transit echo based on the idea of relatively decreasing a resistance component of an external circuit. However, employment of such transformer involves a problem that a transformer per se is expensive and nevertheless bulky, and is therefore unsuited for an electric circuit employing a surface acoustic wave device developed for uses requiring a high degree of miniaturization. Furthermore, employment of a transformer decreases the freedom in circuit designing.
On the other hand, in order to increase the resistance component proper of a transducer, it is known to make short the overlapping length of the adjacent electrode fingers 2a, 2b and 3a, 3b, as shown in FIG. 1, to decrease the number of paired electrodes, and so on. However, reduction of the overlapping length of the adjacent electrode fingers causes deflection or divergence of a surface wave excited by the input transducer 2, for example, whereby it could happen that a portion of the surface wave is not received by the output transducer 3, thereby increasing the deflection loss. On the other hand, reduction of the number of paired electrodes makes it difficult to achieve a complicated freqeuency characteristic. Accordingly, such reduction of the overlapping length of the adjacent electrode fingers and reduction of the number of paired electrodes still involve problems to be solved and these can not be used as an expedient for effectively improving the electric triple transit echo.
One might think of reduction of the output impedance and the input impedance of the external circuit, i.e. the resistance value of the resistance R1 or R2 shown in FIG. 2. However, in general the impedance of such external circuit (the resistance R1 or R2) is primarily determined by the gain required by the whole circuit configuration and it is impossible to properly select such value individually in combination with a surface acoustic wave device SAW. As a result, the freedom in circuit designing of an electric circuit as a whole, including the external circuit, is degraded.